Dealing with phenotypic traits which are the expression of genotypes at a single locus on the X-chromosome, we develop a model of genetic transmission which takes into account a random environmental factor, this random environmental effect being responsible for the incomplete penetrance of the genotypes. In other words a given genotype has no longer a unique phenotypic expression and a given phenotype could be the expression of any genotype at this single locus. We derive the covariance between relatives under X-linked transmission. The covariance between relatives allows us to find, for any model of transmission, the maximum number of parameters that can usefully be fitted when the data are prevalences among relatives of probands.